Conventionally, fiber optic cables (˜15 mm O.D.) were installed in standard underground ducts (1.25 in/32 mm I.D.) by pulling cables through the ducts. This installation procedure caused a significant amount of force to be applied on the cable due to friction inherent in a pulling installation. Thus, a new method for installing fiber optic cables was developed that utilized compressed air blown into the duct to install the cable (i.e., “blown installation”). This procedure is almost stress-free to the cable, as only a minimal force is applied to the cable during installation.
Further, the use of standard ducts and conventional fiber optic cables failed to maximize the volume available in the duct. Thus, a new approach utilizing very small ducts (i.e., “microducts”) has been developed. As shown in FIG. 1, the available space in standard ducts 10 is increased by the provision of microducts 12 (i.e., 12 mm OD) within standard ducts 10. Fiber optic cables 14 sized to fit within microducts 12 (i.e., microcables) are then installed.
FIG. 1 also illustrates one of the benefits of microduct technology, inasmuch as it allows microducts to be installed in ducts that already have existing cables. Thus, in congested urban areas, a duct that was once thought to be full can have more cables installed in it.
Designing and developing a cable optimized for microduct technology involves a balance of several parameters. In addition to standard design parameters of fiber optic cables to meet environmental and physical conditions, the cable must also be designed for the blowing installation in the microducts.
Prior art cables designed for microducts are generally of a central tube type, in which optical fibers are provided in the center of the optical cable. This central tube type cable has been used because it has been the only means by which a high fiber density (e.g., 72 f) could be achieved in a small diameter cable (e.g., 8 mm). It has heretofore been impossible to manufacture a loose tube (i.e., wherein fibers are loose within the buffer tubes) cable small enough to be effectively used within microducts, while still achieving a high fiber density.
The central tube cables have strength members to prevent the cable from “folding over” or buckling within the microduct during installation, which are positioned radially outward from the centrally located optical units. This combination of optical units and strength members are then covered by a jacket, made of a plastic material such as High Density Polyethylene (“HDPE”), and having a smooth outer surface.
However, such central tube type designs suffer many disadvantages. Although the radial strength members provide good axial reinforcement for the optical cable, they also greatly inhibit the ability of the cable to bend during installation. Further, if the strength members are not applied axi-symmetrically, there are preferential bending directions of the cable. These characteristics degrade the ability of such cables to be used in blown installations. Further, the conventional jacket material, along with the jacket's smooth outer surface, impedes the ability to use blown installation, because of the high friction and large surface area contact between the jacket's outer surface and the inner surface of the microduct, and the lack of friction between the smooth jacket and air or other fluid passing over the cable. Lastly, the requirement of axially placed strength members degrades the amount of area available for optical units, and therefore has a corresponding decrease in the cable's optical performance.
The present invention is directed to overcoming one or more of the problems as set forth above.